The application fields of an optical lens set may generally be divided into several categories, one of which has endless image distance and limited (or short) object distance, such as: the lens sets of slide projector, overhead projector, or film projector, etc. Another kind of optical lens set has limited (or short) image distance and endless object distance, such as: the lens sets of camera, video camera, and telescope, etc. There are also two kinds of application mode for the lens set having limited image distance and object distance: one has longer image distance and relatively shorter object distance, for example, the lens set of microscope, and another one has longer object distance and relatively shorter image distance, and the image scanner of charge coupled device is then one application category of this kind of optical lens set. For said application categories of different kinds of optical lens set, there are different characteristics and limitations of design, assembly, and application for each mirror within the lens set. The main objective of the invention is to make an improvement for the optical lens set, of image scanner, having longer object distance but relatively shorter image distance.
Please refer to FIG. 1, which is an embodiment of a typical flatbed optical scanner 1 commonly seen in current market. Wherein, a document window glass 12 is arranged on the upper side surface of the casing 11 of a scanner 1 for supporting a document to be scanned (not shown in the figure) and, through a driving device 13, an optical chassis 14 is brought along to proceed a linear motion along the guiding rod 15 in the hollow casing 11, such that an image scanning job may be executed to the document placed on the glass 12.
Please refer to FIG. 2, which is the A—A cross-sectional view for the optical chassis 14 of the image scanner 1 shown in FIG. 1 according to the prior arts. The optical chassis 14 includes: a hollow casing 141, a light source 142 positioned at an appropriate position on the upper side surface of the casing 141, a light-guiding device comprised by plural reflective minors 143, a lens set 144, and a charge coupled device (abbreviated as CCD). From the light source 142, the light is incident onto the document (not shown in the figure) placed on the glass 12 and, after the reflected light of the image of the document enters the casing 141 of the optical chassis 14, it is reflected and direction-changed by plural reflective mirrors 143 of the light-guiding device for increasing the optical length to an appropriate length, and the image is then focused and formed on the charge coupled device 145, which converts the scanned image data into electric signals that are readable for the computer.
As shown in FIG. 1 and FIG. 2, the reflective mirror 143 of the optical chassis 14 of the prior arts is comprised of a glass piece formed as thin plate and coated with silver, and additional spring piece 146, fixing mechanism, or screw are needed to fix the reflective mirror 143 at a predetermined position on the inner side of the casing 141. Furthermore, since each reflective mirror 143 all has only one reflective plane to proceed light reflection only once so, in order to reach the total track (abbreviated as TT, that is, the total value of Y1+Y2+. . . +Y5, as shown in FIG. 2) needed by the lens set 144 to focus a clear image, the distance and the corresponding angle between each reflective mirror 143 are needed to be arranged appropriately.
As shown in FIG. 3, which is a plane developing illustration to show that an image of the document 16 of a typical image scanner 1 is reflected by the reflective mirror 143 and finally focused on the charge coupled device 145 by the lens set 144. In FIG. 3, when an image scanner 1 proceeds image scanning, the relative relationship of the distances between the document 16 and each optical element is as follows: TT (total track) is the distance between the document 16 and the charge coupled device 145, W is the width of the document 16, the length of the effective pixel range of the charge coupled device 145 is L, the effective focus length (abbreviated as EFL) of the lens set 144 is EFL, the distance between the last lens (the mirror that is most close to the charge coupled device) of the lens set and the charge coupled device 145 is BFL (Back Focus Length). Each value described above is mainly determined by the optical design parameters of the lens set 144.
For the all image scanner 1 known currently, the magnitudes of its TT value and BFL value are still the most important factors influencing the size of the optical chassis 14, while the size and the occupied space during scanning of the optical chassis 14 again occupy over 80% of the entire volume of a traditional flatbed image scanner 1. Since the current trend of the design concept of the electronic information products is still toward the directions of minimization, lightness, thinness, and convenience of being carried personally, so how to further reduce the size of the optical chassis 14 will be the essential key-point for further shrinking the entire volume of the image scanner 1. However, for the all products of the flatbed image scanner 1 seen in current market according to the prior arts, their TT value and BFL value are still relatively large. For example, for a traditional charge coupled device having 600 dpi resolution, 4 μm pixel pitch and capable of scanning the maximum document paper in A4 size, the current optical lens set according to prior arts may only reach the length value of TT value≧240 mm and BFL value≦18 mm. As for the charge coupled device having higher resolution of 1200 dpi, same pixel pitch of 4 μm and capable of scanning the maximum document paper in A4 size, its TT value is further greater than 350 mm. Furthermore, for the charge coupled device, of low level, having lower resolution of 300 dpi, 7 μm pixel pitch and adapted for scanning document in A4 size, its TT value is still necessarily greater than 240 mm. In fact, the TT values of the prior arts described thereinbefore still have large room for further improvement, otherwise they will cause the entire volume of the optical chassis and the image scanner still very large and uneasy to be shrunk, and this shortcoming should be solved as soon as possible.
The patents of the U.S. Pat. Nos. 6,208,474, 6,014,262, 6,208,474B1, 5,386,312, and 6,147,811 had disclosed the structural design and allocation of the mirrors inside the optical lens set of several kinds. However, the prior patents have never disclosed the same technical characteristics and achievable functions as those of the present invention.